| The Yangtze estuary is located at the intersection of ocean and land,and its salinity is affected by the combination of social and economic activities in the Yangtze River Basin,East China Sea and Delta.The Sea Surface Salinity directly reflects the distribution of dilute water in the estuary area.For the study of freshwater plume front,Yangtze River material transport,estuary environment changes,safety of freshwater resources in megacities and so on have important implications.As a new dynamic monitoring technology,satellite remote sensing can provide a wide range of water environment conditions on the SSS in a timely manner,and can analyze the spatiotemporal changes of SSS in combination with long-term sequence satellite data.Optical remote sensing satellites have high spatial resolution and can be applied to small-scale areas such as estuaries and offshore.Based on Landsat 8 OLI,Sentinel-3 OLCI,GF-1 WFV and MODIS satellite data and SSS measured data off Yangtze estuary,the SSS off Yangtze estuary was retrieved,and the remote sensing retrieval method was carried out.Through comparative analysis,choose the most suitable satellite platform and spectral band for the Yangtze estuary SSS retrieval.The retrieval model was applied to remote sensing monitoring of long-term sequence SSS change.The spatial and temporal differentiation of SSS in the Yangtze estuary since 2000 was studied,and the influencing factors of SSS distribution change were analyzed.Concluded as follow:(1)The negative correlation between the SSS and CDOM absorption coefficient of the Yangtze estuary was explored.The correlation coefficient is different between the flood season and the dry season.It is necessary to retrieve the SSS of the flood season and the dry season respectively.The spectral curves of CDOM in the Yangtze estuary were analyzed under different salinities.At the same time,the common bands of CDOM retrieval were combined.350-680 nm was selected as the range of the Yangtze estuary SSS remote sensing retrieval model.(2)The SSS retrieval model of the Yangtze Estuary based on Landsat 8 OLI,Sentinel-3 OLCI,GF-1 WFV and MODIS was constructed.The Landsat 8 OLI flood season retrieval model utilized 443 and 561 nm bands with an RMSE of 1.83 psu.The Sentinel-3 OLCI retrieval model uses the 442.5 and 510 nm bands in the dry season and the RMSE is 1.93 psu;the flood season uses the 442.5 and 560 nm bands,and the RMSE is 1.02 psu.The WFV2 flood season retrieval model of GF-1 utilizes the 557 and 676 nm bands,and the RMSE is 2.14 psu;GF-1 WFV4 flood season retrieval model utilizes the 550 and 696 nm bands,and the RMSE is 1.62 psu.The MODIS dry season retrieval model uses the 469 and 555 nm bands with an RMSE of 0.63 psu;the flood season model uses the 443 and 555 nm bands with an RMSE of 0.69 psu.(3)Compare and analyze the band,retrieval precision and spatial resolution of satellite data,revisit period,historical data,etc.of four satellite data retrieval models,among which MODIS has high spectral resolution,moderate spatial resolution and Adequate long-term sequence data,suitable for SSS retrieval in the Yangtze River estuary,capable of long-term sequence analysis.The correlation between the MODIS retrieval of the Yangtze estuary and the Yangtze river runoff in each month from 2000 to 2016 was analyzed,and the applicability of the retrieval model on the time scale was verified.(4)Analysis of the spatial distribution of SSS off the Yangtze esturay from 2000 to 2016,showing a trend of gradient distribution.The trend of this spatial gradient is significantly affected by the seansonal change of runoff.The seawater tongue can cross the east of 123°E and north of 31°N,reach the vicinity of the north-south branch of the Yangtze estuary,and the high-low salinity seawater boundary line appears in the northern part of the Yangtze estuary.The dilute water in flood season can affect a wider range.The high-salinity seawater is concentrated in the area south of 123°E and south of 31°N,and the area with lower salinity appears in the northern part off the Yangtze estuary.(5)The seasonal variation of the SSS off the Yangtze estuary is consistent with the seasonal variation of the horizontal circulation.In winter,the Taiwan warm current moving northward and the divergent water and the coastal current moving in the southeast direction are deflected to the northwest direction.The remote sensing retrieval analysis shows that the average salinity field off the Yangtze estuary exists in the northwest direction.In the average salinity field off the Yangtze estuary in March,there is a high salinity water tongue that is consistent with the northwest direction and rotates counterclockwise.The two arc-shaped small freshwater areas near 31°N and 30°N correspond to the cyclone nests produced by the Taiwanese warm current through the Zhoushan Islands.In the summer,under the strong runoff of the Yangtze River,the Yangtze river dilute water deflected to the northeast,and the Taiwan warm current also moved to the northeast under the action of strong dilute water.Remote sensing analysis of SSS revealed that the high and low salinity spatial orientation off the Yangtze estuary in August has the same direction as the water flow.The SSS distribution forms a northeast direction boundary in the area east of 123°E south of 31°N.(6)The average salinity of the surface of the Yangtze estuary in 2000-2016 was opposite to runoff in March and August,and the correlation coefficients were-0.61 and-0.80,respectively,and there is also a good negative correlation in interannual variations.The retrieval results show that the average SSS in March showed a slight decline,and the average SSS in August fluctuated greatly,showing an overall upward trend.(7)Analysis of the Yangtze estuary wind field,horizontal flow field and climate anomaly shows an impact on the distribution of SSS in the surface of the Yangtze estuary.The strong southwesterly wind in summer makes the fresh water move to the northeast,and the north of the Yangtze estuary has lower salinity;the prevailing southeast wind The fresh water is deflected to the northwest,and the salinity is increased near 123°E outside the Yangtze estuary;the Taiwan warm current is the source of high salinity seawater,and the Yangtze River dilute water is the source of low salinity.The dilute water and the East China Sea coast flow merge with the Taiwan warm current movement the path of which is an important cause of the SSS distribution in the surface of the Yangtze estuary;the El-Ni?o phenomenon can weaken the circulation and affect the SSS distribution off the Yangtze estuary,but the Yangtze River runoff has a stronger influence. |
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